1. Field of the Invention
The present invention relates to extensible lens-barrels used in, for example, cameras.
2. Description of the Related Art
A typical extensible lens-barrel is disclosed in, for example, Japanese Patent Laid-Open No. 2002-277711. This extensible lens-barrel utilizes a differential extensible mechanism in which a differential cylinder and a rectilinear-regulation part rotate relatively with each other such that lens-barrel components disposed adjacent to the corresponding inner and outer peripheries of the differential cylinder and the rectilinear-regulation part are extended or retracted.
FIGS. 4 to 6 illustrate a digital camera which is provided with the lens barrel disclosed in Japanese Patent Laid-Open No. 2002-277711. In detail, FIG. 4 illustrates the operational state of the camera, that is, the state in which the lens barrel is extended. On the other hand, FIG. 6 illustrates the non-operational state of the camera, that is, the state in which the lens barrel is retracted. FIG. 5 is a sectional view of one of first-lens-holder pins 121a shown in FIG. 4 taken along line 5—5.
Referring to FIGS. 4 to 6, the camera is provided with a camera body 101, a liquid crystal display unit 102, and a cover 103. The liquid crystal display unit 102 and an operating part, which is not shown in the drawings, are included in the cover 103, and the camera body 101 is covered with the cover 103. The camera body 101 includes a viewfinder block 104; a hard substrate 105 having an electrical circuit; a battery; a recording unit; a strobe light; and an extensible lens-barrel 106. The battery, the recording unit, and the strobe light are not shown in the drawings.
The structure of the lens-barrel 106 used for digital cameras of this type will now be described.
The camera is further provided with a CCD (charge-coupled device) 107, i.e. an image-capturing element; a low-pass filter 108; a base 109; a third lens 110; a third-lens holder 111; and a lens moving mechanism 112 which includes a step motor and a thread-rolling unit. The CCD 107 and the low-pass filter 108 are fixed to the base 109. The third-lens holder 111, which supports the third lens 110, and the lens moving mechanism 112 are attached to the base 109. The lens moving mechanism 112 moves the third lens 110 in the optical-axis direction. Furthermore, the camera includes a fixed cylinder 113, a drive ring 114, and a gear array 115. The fixed cylinder 113 is fixed to the base 109, and the inner periphery of the fixed cylinder 113 is provided with a cam-groove 113a and a rectilinear opening 113b. The drive ring 114 is disposed around the fixed cylinder 113 and can rotate with respect to the optical axis. The outer periphery of the drive ring 114 is provided with a gear portion 114a. The drive ring 114 is driven by a lens-barrel driving unit such as a lens-barrel motor, which is not shown in the drawings, and the gear array 115 via the gear portion 114a. Furthermore, the inner periphery of the drive ring 114 is provided with a drive opening 114b. 
The fixed cylinder 113 encloses a cam-cylinder 116 whose outer periphery is provided with a cam-pin 116a and a drive pin 116b. The cam-pin 116a engages with the cam-groove 113a of the fixed cylinder 113, and the drive pin 116b engages with the drive opening 114b of the drive ring 114. Furthermore, the inner periphery of the cam-cylinder 116 is provided with first cam-grooves 116c and second cam-grooves 116d. When the drive ring 114 rotates, the drive pin 116b engages with the drive opening 114b and allows the cam-cylinder 116 to rotate with respect to the optical axis while the cam-pin 116a moves along the cam-groove 113a. 
The camera is further provided with a rectilinear cylinder 117 which is enclosed by the cam-cylinder 116, a first lens 118, a lens barrier 119, a barrier driving unit 120, and a first-lens-holding cylinder 121. The rectilinear cylinder 117 includes a rectilinear key 117a which engages with the rectilinear opening 113b of the fixed cylinder 113. Moreover, with respect to the optical-axis direction, one end of the rectilinear cylinder 117 is provided with bayonets 117b and the other end is provided with a flange 117c such that the cam-cylinder 116 is disposed between the bayonets 117b and the flange 117c. Thus, the rectilinear cylinder 117 can move in the optical-axis direction, but does not rotate with the cam-cylinder 116. The rectilinear cylinder 117 is further provided with three first rectilinear openings 117d which extend through the cylinder wall, and three second rectilinear openings 117e which also extend through the cylinder wall. The lens barrier 119 is for protecting the first lens 118 when the lens barrel is in its retracted state. The barrier driving unit 120 is for opening and closing the lens barrier 119. The lens barrier 119 and the barrier driving unit 120 are fixed adjacent to the front of the first-lens-holding cylinder 121.
The first-lens-holding cylinder 121 includes three pins 121a which engage with the corresponding first cam-grooves 116c of the cam-cylinder 116, and three rectilinear keys 121b which engage with the corresponding first rectilinear openings 117d of the rectilinear cylinder 117. The first-lens-holding cylinder 121 is only permitted to move rectilinearly by the rectilinear cylinder 117, and therefore, does not rotate with the cam-cylinder 116. Thus, the pins 121a slide along the first cam-grooves 116c of the cam-cylinder 116 such that the first-lens-holding cylinder 121 moves forward or backward in the optical-axis direction.
The camera is further provided with a second lens 122, a second-lens holder 125, a shutter blade 123, and a shutter driving unit 124. The second lens 122 is supported by the second-lens holder 125. The shutter blade 123 and the shutter driving unit 124 are attached to the second-lens holder 125. The second-lens holder 125 includes two cam-pins, which are not shown in the drawings, a bias pin 127, and rectilinear keys 125b. The two cam-pins engage with the corresponding second cam-grooves 116d of the cam-cylinder 116. The bias pin 127 is biased outward against one of the second cam-grooves 116d in the radius direction of the lens barrel by a spring 126. The rectilinear keys 125b engage with the corresponding second rectilinear openings 117e of the rectilinear cylinder 117.
The second-lens holder 125 is permitted to move only in the optical-axis direction by the second rectilinear openings 117e, and does not rotate with the cam-cylinder 116. This means that the second-lens holder 125 does not rotate with respect to the camera body 101 but moves forward or backward in the optical-axis direction with the cam-cylinder 116 by engaging with the second cam-grooves 116d. 
The circuit of the hard substrate 105 is electrically connected to the lens-barrel driving unit, the lens moving mechanism 112, and the shutter driving unit 124 via a flexible printed substrate 128.
In the digital camera provided with such a differential extensible lens-barrel, when the operating part in the cover 103 is operated for switching the camera from a non-operational state in FIG. 6 to an operational state in FIG. 4, the lens-barrel driving unit is activated via the circuit on the hard substrate 105. Thus, the drive ring 114 is rotated such that the cam-cylinder 116 and the first-lens-holding cylinder 121 are driven forward or backward in the optical-axis direction. Moreover, the second-lens holder 125 moves outward in the optical-axis direction. Furthermore, the lens moving mechanism 112 is also activated to move the third lens 110. Accordingly, the optical components are positioned to a state shown in FIG. 4.
When the shutter is released in the operational state, the camera performs automatic focusing and automatic exposure, and then processes and records the image.
FIG. 8 illustrates a broken lens-barrel caused by, for example, dropping the camera when the lens barrel was in an extended state. More specifically, the front surface of the lens barrel, for example, was subjected to a large impact when the camera was dropped.
The front surface of the lens barrel was subjected to an impact as indicated by an arrow in FIG. 8. This impact caused one of the pins 121a of the first-lens-holding cylinder 121 to disengage from the corresponding first cam-groove 116c. 
According to Japanese Patent Laid-Open No. 2002-277711, as shown in FIG. 7, each second rectilinear opening 117e of the rectilinear cylinder 117 is provided with claw portions 117f and 117g which extend toward each other in the circumferential direction of the lens barrel. Moreover, the first-lens-holding cylinder 121 has rectilinear guide grooves, i.e. rails 121c and 121d, which are engageable with the claw portions 117f and 117g, respectively, in the radius direction of the lens barrel. The rails 121c and 121d extend in the optical-axis direction. Consequently, the claw portions 117f and 117g and the rails 121c and 121d prevent the displacement of the cam-follower, i.e. the pin 121a, from the cam-groove 116c. This means that even if an impact against the first-lens-holding cylinder 121 imparts a force against the cam-follower, the cam-follower is prevented from disengaging from the cam-groove 116c in the radius direction of the lens barrel. Accordingly, this prevents the lens barrel from breaking.
With such a structure, however, there are cases where a large impact may force the cam-follower, i.e. the pin 121a, to become wedged into the cam-groove 116c, causing the lens barrel to break.
As a countermeasure to this problem, it is possible to use a harder material for the cam-cylinder 116, or to provide a greater depth for the cam-grooves 116c so that larger portions of the pins 121a can be embedded in the cam-grooves 116c. 
A typical lens-barrel used for cameras is normally formed of engineering plastics, such as polycarbonate. The reason for this is that engineering plastics have a suitable weight to strength ratio, and moreover, have high flexibility in view of providing complex structures. Furthermore, engineering plastics contribute to cost reduction. Generally, for increasing the strength, pellets formed of, for example, carbon fiber or glass fiber are added to the plastic. These additives, however, cause problems and are not desirable due to the fact that they lower the reliability of the mold used for the fabrication of the lens barrel. On the other hand, providing a greater depth for the cam-grooves 116c increases the wall thickness of the cam-cylinder 116 and leads to a larger outer diameter of the cam-cylinder 116. This results in the entire lens barrel having a larger outer diameter, and thus goes against the trend in recent years of size and weight reduction.